where \({{\bf {x}}_k}=(x_{1,k},\cdots ,x_{M,k})^{\rm T}\)  is the signal vector intended for cluster \(k\) , \(\zeta _{m,k}\)  stands for the power allocation coefficient of signal \(x_{m,k}\) , which is subject to the normalization constraint \(\sum \nolimits _{k = 1}^K {{\zeta _{m,k}}} = {1}/{M}\)  for all \( m \in [1,M]\) , to ensure the fairness among different data streams. On the other hand, the application of NOMA can guarantee clusters' fairness by taking advantage of the difference in channel qualities between the BS and clusters[1]}.
